An exhaust aftertreatment device that purifies nitrogen oxides (NOx) contained in exhaust gas of an engine with a selective catalytic reduction (abbreviated as “SCR” hereinafter) has been known. Urea aqueous solution injected by an injector is supplied to the SCR. The injector is attached as a component of a mixing device provided upstream of the SCR (see, for instance, Patent Literature 1). The urea aqueous solution is injected from the injector to exhaust gas flowing through the mixing device to mix the urea aqueous solution with the exhaust gas within the mixing device. As a result, the urea aqueous solution is thermally decomposed by the heat of the exhaust gas to produce ammonia. The ammonia is used as a reductant in the SCR.
Some of the mixing devices are provided with a block tapered member between the injector and a pipe to which the injector is attached so as to keep the injector from being influenced by the heat of the exhaust gas. The tapered member has a recess defined by a tapered face and an injection nozzle of the injector is exposed at a depth side of the recess (see, for instance, Patent Literature 2). However, with the use of such a block member, though the thermal influence of the exhaust gas can be reduced, since the injection nozzle is located remote from an exhaust pipe and the depth of the recess is increased, the exhaust gas becomes less likely to flow around toward the injection nozzle, so that drops of the urea aqueous solution are likely to be resided in the recess. When the resided urea aqueous solution is crystallized to be deposited, the injection of the urea aqueous solution from the injection nozzle is disturbed.
Thus, it has been proposed to use a plate-shaped heat insulation material instead of the above thick block tapered member (see, for instance, Patent Literature 3). According to the disclosure of Patent Literature 3, since the injection nozzle of the injector can be provided substantially flush with an opening of an exhaust pipe, the urea aqueous solution resided around the injection nozzle can be thermally decomposed by the heat of the exhaust gas, thereby restraining the deposition of the urea aqueous solution. In addition, since the part around the injection nozzle is covered with the heat insulation material, the thermal influence of the exhaust gas to the injector can also be restrained.